1. Field of the Invention
The present invention relates generally to techniques for operating mechanical instrument gauges using digital information derived from measurements of physical quantities broadcast over a serial communications link, and more particularly, to improved techniques which permit interfacing analog gauges to a digital communication system that broadcasts measurands as numbers during defined intervals over a serial data link to intelligent devices that share time on the link, the intelligent devices have circuits that use measurands as numbers for controlling the operation of the analog gauges.
2. Description of the Prior Art
In prior art vehicle serial digital communications systems, digital signals control digital speedometers very effectively by processing numerical data representing measured physical quantities as disclosed in U.S. Pat. No. 4,779,213 which issued Oct. 18, 1988 to the present inventor, William Luitje.
In vehicles equipped with serial data links for data communication, intelligent devices use the same sensed measurands for updating digital instrument clusters; however, having just digital instrument clusters working with serial data links, restricts the buying public's choice of display devices. Many consumers, who prefer analog instrument clusters, may desire to own a vehicle equipped with advanced digital electronic features that utilize digital data transmission links but do not want the digital electronic instrument cluster which accompanies it. Some desire to replace the digital instrument cluster with an analog mechanical instrument cluster. But due to the lack of compatibility, an analog mechanical instrument cluster usually cannot directly replace the digital electronic instrument cluster. Although having separate data lines may permit effecting the substitution, this increases cost, design and manufacturing complexity.
Additional problems exist when trying to substitute an analog gauge for a digital gauge in a vehicle that utilizes a digital data communications system for controlling an instrument panel. To use digital signals to drive electrically controlled analog gauges, some form of digital to analog (D/A) converting device must provide the interface between the digital input signals and the analog gauge movement. Such converting device may take the form, e.g. of H-drive circuits having logic controlled current sink and source devices for driving gauge coil windings appropriately to effect correlative gauge indications.
In vehicles that use digital communications systems, transmission of number representation of measurands seldom occurs continuously. This is because the amount of information that can be communicated over the link is limited by the bandwidth of that link. Other modules in the system also need to use the link to communicate with each other. Also, each module has other tasks to accomplish and cannot spend all its time communicating. For example, in the process of measuring speed, electrical pulse signals from a distance sensor become interrupt signals of a measuring computer. The measuring computer counts the number of distance sensor pulses within a discrete interval to determine vehicle speed. Then the measuring computer normally broadcasts the speed information over a data link to other computer controlled devices on the link such as an electronically controlled instrument cluster at a predetermined time interval.
At this point, a problem becomes apparent. If, e.g., the speed changes between data transmissions, gaps develop in the information transmitted to the speedometer. Such a gap, if allowed to persist, would cause the gauge needle to jump from one position to the next almost instanteously and then oscillate briefly due to the under-damped quality of the gauge. This sort of needle movement is hard to read, distracting and differs from what people are accustomed to in an automotive instrument gauge; hence, it is totally unacceptable.
Hence, a search took place to find means which would cause analog gauge needle deflections to behave normally in a digital control system setting yet provide accurate meter indications of the measurands. This search resulted in the distributed data measuring, communications, and display system that embodies the principles of the present invention.